1. Field of the Invention
The present invention relates to an ultrasonic receiving apparatus to be used for obtaining ultrasonic images by receiving ultrasonic waves.
2. Description of a Related Art
Recently, in order to obtain high quality three-dimensional images using ultrasonic waves, development of a two-dimensional sensor capable of obtaining two-dimensional images without mechanically shifting a sensor array is proceeding.
Conventionally, as an element (vibrator) used for transmitting and receiving ultrasonic waves, a piezoelectric element that includes piezoelectric ceramic represented by PZT (Pb (lead) zirconate titanate) or macromolecule piezoelectric material represented by PVDF (polyvinylidene difluoride) has been generally used. However, in the case where the two-dimensional array is fabricated by using these elements, since micro-processing on elements and wiring to a large number of micro-elements are required, it is difficult to achieve further miniaturization and integration of elements. Even though the difficulties could be overcome, such problems still remain that crosstalk between elements increases, electrodes of microelements become easily broken, and SN-ratio becomes lower due to increase of electric impedance caused by micro-wirings. Consequently, it is difficult to apply the two-dimensional sensor array using PZT or PVDF in practice.
In order to avoid such problems, also a photo-detection type ultrasonic sensor is under study in which a received ultrasonic wave signal is converted into an optical signal and then detected. As the photo-detection type ultrasonic sensor, a sensor in which a fiber Bragg grating (abbreviated as FBG) is used (for example, TAKAHASHI et al. (National Defense Academy) “Underwater Acoustic Sensor with Fiber Bragg Grating”, OPTICAL REVIEW Vol. 4, No. 6 (1997), pp. 691–694), and a sensor in which a Fabry-Perot resonator (abbreviated as FPR) structure is used (for example, UNO et al. (Tokyo Institute of Technology) “Fabrication and Performance of a Fiber Optic Micro-Probe for Megahertz Ultrasonic Field Measurement”, T.IEE Japan, Vol. 118-E, No. 11, 1998, pp. 487–492) are reported. Fabricating a two-dimensional sensor array by using such ultrasonic sensor provides the advantages that electrical wiring to a large number of microelements is not required and good sensitivity can be obtained.
Further, as a photo-detection type ultrasonic sensor having a two-dimensional detection surface, it is also proposed that a polymer film having a Fabry-Perot structure is used for detecting ultrasonic waves (Beard et al. (University College London) “Transduction Mechanisms of the Fabry-Perot Polymer Film Sensing Concept for Wideband Ultrasound Detection”, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 46, NO. 6, NOVEMBER 1999, pp. 1575–1582). In the film-like ultrasonic sensor, since processing on a large number of microelements is not required, the cost can be reduced. The photo-detection type ultrasonic sensor utilizes an ultrasonic detecting element having optical reflectance characteristics that change by receiving ultrasonic waves.
However, in such ultrasonic detecting element, the detection sensitivity widely varies since the optical reflection characteristics change due to changes in temperature and humidity. Further, in the ultrasonic detecting element having a two-dimensional detection surface, the detection sensitivity varies since the optical reflection characteristics differ depending on the respective positions on the detection surface. As described above, the problem in practical use of the photo-detection type ultrasonic sensor is how to control changes in detection sensitivity caused by environmental factors such as temperature and structural factors. For this purpose, a conceivable solution is, for example, to adjust the wavelength of the light outputted from the light source to the point where the sensitivity of the ultrasonic detecting element is high. However, it is difficult to tune the wavelength of the light outputted from the light source because the reflection characteristics change very steeply. Another conceivable solution is to allow broadband light to enter the ultrasonic detecting elements having different reflection characteristics depending on the positions and to separate the reflected light by filtering it. In this case, however, there are problems that the constitution of the ultrasonic detecting element becomes complicated, and the cost rises. Yet another conceivable solution is to vary the reflection characteristics in respective plural detection areas of the ultrasonic detecting element. Also in this case, however, the constitution of the ultrasonic detecting element becomes complicated and the cost rises.